Revenue (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Fixed-price Contract [Member]
Disaggregation of Revenue [Line Items]
Performance obligation	$ 485
Percentage of revenue expected to be recognized by set date	80.00%
Revenue, remaining performance obligation, expected timing of satisfaction, year	2019
Revenue, remaining performance obligation, expected timing of satisfaction, explanation	We expect to recognize approximately 80% of this revenue through 2019, with the remainder recognized thereafter.
Index Priced Contracts [Member]
Disaggregation of Revenue [Line Items]
Performance obligation	$ 450
Percentage of revenue expected to be recognized by set date	10.00%
Revenue, remaining performance obligation, expected timing of satisfaction, year	2019
Revenue, remaining performance obligation, expected timing of satisfaction, explanation	We expect to recognize approximately 10% of this revenue through 2019, with the remainder recognized thereafter.
Sales Revenue, Net [Member] | Indiana [Member]
Disaggregation of Revenue [Line Items]
Percentage of revenue	74.00%	71.00%